The loops of Henle and the vasa recta function in the process of water conservation by maintaining high salt and urea concentrations (low water concentration) in the kidney medulla. The collecting duct is the final site in the kidney where either a concentrated or dilute urine can be produced. To retain water the permeability of the collecting duct cell membrane is increased by antidiuretic hormone such that water moves passively from the duct lumen (high water concentration) into the medulla (low water concentration). The measurement of cell membrane water permeability and studies on the control of that permeability and the mechanisms of cell volume regulation in collecting ducts are extremely important for understanding the function of these renal structures.